Global Navigation Satellite Systems (GNSS) is a collective term for a variety of satellite navigation systems which use orbiting satellites as navigation reference points to determine position fixes on the ground. GNSS includes the Global Positioning System (GPS), the Global Navigation Satellite System (GLONASS), Galileo, the European Geostationary Navigation Overlay (EGNOS), Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS), the Wide Area Augmentation Service (WAAS), and the Compass system. GNSS is increasingly used in a wide variety of applications including surveying.
In typical civilian applications, a single GNSS receiver can measure a ground position with a precision of about ten meters. This is, in part, due to various error contributions which often reduce the precision of determining a position fix. For example, as the GNSS signals pass through the ionosphere and troposphere, propagation delays may occur. Other factors which may reduce the precision of determining a position fix may include satellite clock errors, GNSS receiver clock errors, and satellite position errors (ephemeredes). One method for improving the precision for determining a position fix is called Real-Time Kinematic (RTK) GNSS.
FIGS. 1A, 1B, and 1C illustrate steps performed initiating and operating an exemplary conventional RTK system. In FIG. 1A, an RTK base station 105 is disposed above a known location 101. Typically, the geographic coordinates (e.g., latitude, longitude, and elevation) known location are surveyed with a high degree of precision. Also, a typical RTK base station (e.g., 105) comprises a GNSS antenna, GNSS receiver, and a wireless broadcasting data link.
To initiate RTK base station 105, the power is initialized and RTK base station 105 attempts to determine its geographic location (e.g., the geographic coordinates of known location 101). However, due to the combination of error contributions discussed above, the precision with which RTK base station 105 can determine its own geographic position is within a 10 meter radius (e.g., 115 of FIG. 1B) of the geographic coordinates of known location 101.
As shown in FIG. 1B, even though RTK base station 105 is in fact disposed above known location 101, it erroneously determines its geographic location to be at position 103. To overcome this known error, an operator communicatively couples a GIS data collector (e.g., 111) to RTK base station 105 and programs in the geographic coordinates of known location 101, as well as the antenna height of RTK base station 105. RTK base station 105 then broadcasts the geographic coordinates of known location 101 and the epoch by epoch raw range observations to each visible satellite.
Referring now to FIG. 1C, when in operation, RTK base station 105 periodically broadcasts the geographic coordinates of known location 101 (e.g., once a minute) rather than the geographic coordinates that it independently derived, as well as the epoch by epoch raw range observations to each visible satellite (e.g., once a second). A surveyor then disposes an RTK rover unit 120 at a position 125 to be measured. In order to initiate RTK rover unit 120, the operator must: initialize power to the unit and then, using GIS data collector 111, program RTK rover unit 120 to listen for the broadcast geographic coordinates of RTK base station 105 as well as the observed raw range observations to each visible satellite.
To determine the geographic coordinates of position 125, the operator uses GIS data collector 111 to query RTK rover unit 120 for the geographic coordinates of position 125. RTK rover unit 120 then uses the epoch by epoch observations sent by RTK base station 105 and combines them with its own observations of raw ranges to each of the satellites. RTK rover unit 120 then precisely calculates the relative differential vector 130 using the raw ranges from the base station to each satellite combined with the raw range from the rover to each of the same satellites. RTK rover unit 120 then determines the geographic coordinates of position 125 using the relative baseline vector 130 and the broadcast reference position. In other words, RTK rover unit 120 calculates the latitude, longitude, and elevation of position 125 by adding relative baseline vector 130 to the broadcast base position (e.g., 101) and then correcting for the height of the antennas above the ground.
In an alternative technique, RTK base station 105 is set up at a location and performs a position fix. RTK base station is then configured to broadcast this position. RTK rover unit 120 uses the position broadcast by RTK base station 105 to calculate its own position. However, because the precise geographic coordinates have not been programmed into RTK base station 105, there is a degree of error associated with the position broadcast by RTK base station 105, as well as the position calculated by RTK rover unit 120 based upon the broadcast position of RTK base station 105. RTK rover unit 120 must at some point occupy a known position, perform a position fix, and determine a correction between the geographic coordinates of the known position and the position fix it performed. RTK rover unit 120 then applies this correction to previously measured points to improve the accuracy of determining the geographic coordinates of those points. It is important to note that in this technique, RTK rover unit 120 continues to use the broadcast position from RTK base station 105 in order to determine it position at a given point. It simply applies the correction, once it has been determined, to that position. Thus, the position at which RTK base station 105 is set up becomes a “fixed” geodetic coordinate for a project. In order to get accurate coordinates, the correction must be applied to position fixes made using the broadcast by RTK base station 105, a second position for RTK base station 105 may not be used.
Among the drawbacks of initiating and operating an RTK system as described above is the amount of time needed to properly program both RTK base station 105 and RTK rover unit 120. For example, the operator must wait for RTK base station 105 to initialize and generate position fix 103. The operator then has to communicatively couple GIS data collector 111 with RTK base station 105 and program the geographic coordinates of known position 101. The operator also has to communicatively couple GIS data collector 111 with RTK rover unit 120 and program it to listen for the broadcast data from RTK base station 105. In addition to the time required for these steps to be performed, there is also the possibility of inadvertent operator error in programming the geographic coordinates of known position 105.
Additionally, there are often problems with communicatively coupling GIS data collector 111 with RTK base station 105 and/or RTK rover unit 120. For example, GIS data collector 111 may use a serial cable connection to communicatively couple with either base station 105, or with RTK rover unit 120. However, the serial cables used to communicatively couple GIS data collector 111 with base station 105, or RTK rover unit 120 are prone to breaking, particularly the pins in the cable connectors. Due to the relatively high cost of these cables (e.g., around $200 each) it is not likely that the operator carries spare cables as a backup.
Alternatively, GIS data collector 111 may utilize a wireless system (e.g., the Bluetooth® system) to communicatively couple with base station 105 and/or RTK rover unit 125. It is sometimes difficult to establish a Bluetooth® connection between GIS data collector 111 and RTK base station 105 and/or RTK rover unit 125. It is also sometimes difficult for GIS data collector 111 to establish a Bluetooth® connection with two separate devices (e.g., RTK base station 105 and RTK rover unit 120). Additionally, sometimes the operator may erroneously re-program RTK base station 105 when attempting to program RTK rover unit 120 if the proper procedure is not followed. Finally, it is not uncommon for RTK base station 105 to utilize a serial cable connection while RTK rover unit 120 to utilize a Bluetooth® connection, thus complicating the initiating procedure.